SHRIRAM SPARK

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LM317 ADJUSTABLE VOLTAGE REGULATOR

lm317

Every project needs a power supply. As 3.3volt logic replaces 5volt systems, we’re reaching for the LM317 adjustable voltage regulator , rather than the classic 7805 . We’ve found four different hobbyist-friendly packages for different situations.

A simple voltage divider  (R1,R2) sets the LM317 output between 1.25volts and 37volts; use this handy LM317 calculator  to find resistor values. The regulator does its best to maintain 1.25volts on the adjust pin (ADJ), and converts any excess voltage to heat. Not all packages are the same. Choose a part that can supply enough current for your project, but make sure the package has sufficient heat dissipation properties  to burn off the difference between the input and output voltages.

Voltage regulator

Schematic of LM317 in a typical voltage regulator configuration, including decoupling capacitors to address input noise and output transients.

The LM317 has three pins: Input, output, and adjustment. The device is conceptually an op amp (with a relatively high output current capacity). The inverting input of the amp is the adjustment pin, while the non-inverting input is set by an internal bandgap voltage referencewhich produces a stable reference voltage of 1.25V.

resistive voltage divider between the output and ground configures the op amp as a non-inverting amplifier so that the voltage of the output pin is continuously adjusted to be a fixed amount, the reference voltage, above that of the adjustment pin. Ideally, this makes the output voltage:

Vout = Vref(1 + RL/RH)

Because some quiescent current flows from the adjustment pin of the device, an error term is added:

Vout = Vref(1 + RL/RH) + IQRL

To make the output more stable, the device is designed to keep the quiescent current at or below 100µA, making it possible to ignore the error term in nearly all practical cases.

Specifications

Attribute Value
Vout range 1.25 V – 37 V
Vin – Vout difference 3 V – 40 V
Operation ambient temperature 0℃ – 125℃
Output Imax less than 1.5 A (assuming factory-suggested heat sinking)
Minimum Load Current no more than 10 mA

Here is a breakdown of the voltage regulators illustrated above:

IC1 LM317LZ  200mA, TO-92   – This is the smallest common LM317 voltage regulator. The part linked can supply 200mA, but 100mA is more common. The TO-92 package can get searing hot because it doesn’t dissipate much heat.

IC2 LM317T  1.5amps, TO-220  – At 1.5amps, this regulator supplies enough power for most digital circuits. We prefer the surface-mount D2Pack version (IC4) because we don’t like to drill holes. The TO-220 package dissipates a ton of heat, and the metal tab will accommodate a heat sink if you want even more cooling. Use this package if you need maximum heat dissipation.

IC3 LM317MDCYR  500mA, SOT-223  – This is our favorite LM317 package. 500mA is plenty of power for many projects, and the small SOT-223 package fits about anywhere.

IC4 LM317D2T  1.5amps, D2Pack  – We design with the D2Pack regulator when a circuit uses more than 400mA of current. D2Pack is a surface-mount version of TO-220 that’s easy to solder.

 

1 Comment

  1. Great blog! I like it a lot!

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